It is known that the method generally used for determining the longitudinal position of the centre of gravity of an aircraft consists in weighing the aircraft on the ground, in measuring the quantities of fuel in the various tanks, in measuring or estimating the passenger and freight loads and in adding these data on a weight and balance chart. Each of these data may be permanently plotted on said chart to follow the evolution of the position of the centre of gravity. These operations are effected either manually or with the aid of a computer which takes into account the starting data and permanently plots in the course of the flight the variations in mass due to the reduction and transfer of fuel.
This known method, which seems to be the only one used at the present time, gives good results insofar as all the data entered are correct, as the transfers have been correctly plotted and as there were no operational contingencies in the various transfer, measuring or calculation systems.
However, it does not make it possible to make a resetting to zero in flight in order to trigger off a fresh process of measuring, in the event of one of the elements having been used for obtaining the measurement being in doubt. This defect is inherent in any system referring to data integrated from the ground measurements.
This known method is therefore limited in reliability and especially in safety.
Up to the present time, for wide-body aircraft, this drawback was hardly inconvenient and only the ground measurement and calculations, before take-off, were important.
In fact, such aircraft could be found only in a stable configuration, i.e. in no case did the centre of gravity risk being located to the rear of the point of manoeuvring, at their departure; further, these aircraft could not dangerously fluctuate, in flight, from this configuration of stability, due to the situation of the different tanks and the sequences of filling and emptying studied so that the aircraft in flight remains within the certified centering limits.
However, on the supersonic aircraft CONCORDE, the problem has been raised and solved in a conventional manner thanks to redundant alarm systems and computers. In fact, to maintain the aircraft easily manoeuvrable in supersonic flight, and therefore to maintain a small distance between the centre of gravity and the point of manoeuvring (defined as the point where the elevator has an infinite efficiency on the load factor of the aircraft), transfers of fuel are effected towards the rear at the moment of passage into supersonic flight; and, inversely, upon return to subsonic flight, a reverse transfer is effected in order not to risk instability. However, in this particular case, being given the operation effected (transfer of fuel in one sole operation), the system is hardly complex.
Similarly, on the wide-body aircraft AIRBUS A 310-300, which presents an adjustable horizontal stabilizer, an additional fuel tank is installed in said adjustable horizontal stabilizer in order to increase the radius of action and to improve the performances by displacement of the centre of gravity towards the rear.
In that case, in order to effect calculation of the longitudinal position of the centre of gravity and to monitor the transfer of fuel so as to maintain minimum the distance between the centre of gravity and the point of manoeuvring, whilst maintaining the stability of the aircraft, a centering computer permanently manages the transfers of fuel. This computer uses the known method, recalled hereinabove, based on the sum of the data.
Furthermore, it is known that the modern technique of aircraft construction is developing towards the production of aircraft with artificial stability, i.e. in which the centre of gravity lies to the rear of the point with respect to which the moment of the aerodynamic forces is independent of the incidence, which necessitates permanently effecting corrections by acting on the elevator at a high frequency.
In such aircraft with artificial stability, the problem of determination of the position of the centre of gravity is therefore acute, as this position must be known in a reliable and precise manner in order to be able to be permanently monitored to ensure flight safety of these aircraft.